For example, as power supply units for chargers, small switching supply units have been generally used each of which includes an output voltage detection circuit, a constant current control circuit, and a photocoupler on the secondary side to realize constant voltage control and a constant current drooping characteristic.
Further, a switching power supply further reduced in size is proposed which does not include an output voltage detection circuit, a constant current control circuit, and a photocoupler on the secondary side and realizes the functions of the circuits and the photocoupler on the primary side (for example, see European Patent Application Publication No. 1211794).
In this conventional switching power supply, the timing of the end of secondary current passing through the secondary winding of a transformer (the off timing of the secondary current) is detected by detecting the timing of the falling edge of a flyback voltage generated on the auxiliary winding of the transformer, so that a period during which the secondary current passes through the secondary winding (the on period of the secondary current) is detected. The secondary current starts flowing when a switching element is turned off. And then, the conventional switching power supply generates a signal (secondary duty control signal) V2_on indicating the on period of the secondary current having been detected thus and controls the oscillation frequency (switching frequency) of the switching element such that the signal V2_on has a constant duty ratio D2on′. Thus the secondary current has a constant duty ratio D2on and a constant current drooping characteristic is realized.
The duty ratio D2on of the secondary current is expressed by the following equation:D2on=t2on/T where t2on represents the on period of the secondary current and T represents the oscillation period of the switching element.
Referring to FIG. 6, operations in the constant current region of the conventional switching power supply will be described below. FIG. 6 is a timing chart showing signal waveforms in the constant current region of the conventional switching power supply. FIG. 6 shows, from the top, the waveform of a drain current Ids passing through the switching element, the waveform of a flyback voltage TR generated on the auxiliary winding, the waveform of the secondary duty control signal V2_on, and the waveform of a secondary current I2p. 
The conventional switching power supply generates the secondary duty control signal V2_on by detecting the timing of a falling edge on the waveform of the flyback voltage TR generated on the auxiliary winding. The secondary duty control signal V2_on is at a high level from when the waveform of the drain current Ids is turned off to when the waveform of the flyback voltage TR falls, and is at a low level from when the waveform of the flyback voltage TR falls to when the switching element is subsequently turned on and is turned off again. And then, the conventional switching power supply controls a switching frequency so as to keep constant the duty ratio D2on′ of the secondary duty control signal V2_on, so that the duty ratio D2on of the secondary current I2p is also kept constant.
The duty ratio D2on′ is expressed by the following equation:D2on′=tron/T where tron represents a period from when the waveform of the drain current Ids is turned off to when the waveform of the flyback voltage TR falls, and T represents the oscillation period of the switching element.
However, only by controlling the oscillation frequency of the switching element so as to keep D2on′ constant, a constant current drooping characteristic cannot be realized with high accuracy. The reason why will be described below.
The flyback voltage TR generated on the auxiliary winding starts declining after the secondary current I2p stops flowing, and has a gradually changing waveform because the waveform is a resonance waveform of the inductance of the transformer and the capacitance of the switching element. Further, a set voltage for detecting the timing of the falling edge of the flyback voltage TR is generally set low in response to fluctuations in input voltage. Therefore, a detection delay time Δtr occurs from when the secondary current I2p stops flowing to when the timing of the falling edge of the flyback voltage TR is detected.
The detection delay time Δtr is kept constant regardless of the output. The time delay Δtr is defined by an inductance L of the transformer and a capacitance C between the drain and source of a power MOSFET composing the switching element, and is proportionate to (LC)1/2. On the other hand, in the constant current region for controlling the switching frequency so as to keep D2on′ constant, the switching frequency increases with an output voltage. Therefore, in the constant current region, as the output voltage increases, the duty ratio D2on of the secondary current I2p is reduced by the influence of the detection delay time Δtr. Consequently, the output current decreases.
FIG. 7 shows the output characteristics of the conventional switching power supply. As shown in FIG. 7, in the conventional switching power supply, as an output voltage Vo increases in the constant current region, the switching frequency is increased and an output current Io is reduced by the influence of the detection delay time Δtr. Conversely, as the output voltage Vo decreases, the switching frequency decreases, the influence of the detection delay time Δtr diminishes, D2on comes close to D2on′, and the output current Io increases.
As described, even when D2on′ is controlled to a constant ratio, the duty ratio D2on of the secondary current I2p changes with a load. Thus the output current Io also changes with a load and a constant current drooping characteristic cannot be obtained with high accuracy.
Higher switching frequencies are effective for the size reduction of a switching power supply. However, as described, the influence of the detection delay time Δtr cannot be eliminated and a constant current characteristic with high accuracy is harder to obtain at higher frequencies in the conventional switching power supply. For this reason, it is difficult to reduce the size of the conventional switching power supply.